Subtract two variables in SHELL

 https://unix.stackexchange.com/questions/93029/how-can-i-add-subtract-etc-two-numbers-with-bash

echo "First number please"
read num1
echo "Second number please"
read num2
echo "Operation?"
read op

but then all my attempts to add the numbers fail:

case "$op" in
  "+")
    echo num1+num2;;
  "-")
    echo `num1-num2`;;
esac

Run:

First number please
1
Second mumber please
2
Operation?
+

Output:

num1+num2

...or...

echo $num1+$num2;;

# results in: 1+2